A variety of industrial and commercial applications require bonding surfaces together and, in certain instances, flat surfaces having relatively large areas. Included in these applications are microelectronics applications for bonding liquid crystal display assemblies and especially relatively large liquid crystal display assemblies and heat-sink attachments, and such commercial applications as windowpane glass and auto windshield applications. When laminating or bonding flat surfaces with a liquid adhesive, the ever present problem of air entrapment requires special attention. For instance, even though a surface may appear flat or planar, small topographic variations allow contacting at multiple points during mating. As the adhesive spreads from such multiple points, the advancing fronts can meet and thereby trap pockets of air.
The larger the area of the mating surface, the higher the incidence of air entrapment. Furthermore, the lower the viscosity of the adhesive, the higher the incidence of air entrapment.
Trapped air, depending upon the desired product, presents problems of varying degrees. For instance, when dealing with bonding large liquid crystal display assemblies, the individual LCD tiles are arranged in a matrix and secured to a tile carrier. The tile carrier typically includes a cover plate and a back plate with the LCD tiles sandwiched between them. The bonding of the back plate and cover plate to the liquid crystal display tiles should be as void free as possible. In order to achieve a void free bond, proper dispensing of the adhesive mass along with providing a pattern that allows spreading out from the center outward and sweeping air out as the front advances must be achieved. In addition, the surfaces to be bonded must be mated parallel to each other. It is also desirable that the point contact of the mating surfaces with the adhesive between them be controlled and that the pattern employed permit complete coverage of the surface area of the mating substrates regardless of shape such as rectangular, square or polygon. Also, it is necessary to control the bond line.
With respect to these requirements, the proper dispensing of the adhesive mass can be readily achieved employing metered dispense units well known in the art. Moreover, it has previously been determined that an X pattern extending the entire diagonal length of the surfaces to be bonded is necessary for achieving complete coverage. Furthermore, a majority of the adhesive should be dispensed in the center of the adhesive pattern since spreading is initiated in the center, and spreads out radially. Nevertheless, it has been found that regardless of the pattern geometry in the center, e.g. circular, elliptical, square, smaller scale X pattern and the like, the geometry of the spreading area quickly reverts to circular or elliptical. However, even when employing an X pattern with the diagonal spokes extending all the way to the corners of the surface to be bonded, a void free bond line is not necessarily achieved.